[tjoff]

I have tons of sites opened 24/7. If yours happen to be loaded and you try to exploit my machines resources I will be quite pissed.

[kristiandupont]

That's fair.

I suppose a more polite version would run only when the tab is in focus. But as has been pointed out, this doesn't seem feasible because of the tiny amount of value generated so I think it's an interesting idea to explore more than anything else.

[mixedbit]

Browsers throttle not active tabs.